1. Field of Invention
This invention relates to measuring and testing circuits in general and in particular to a circuit responsive to an electrothermal sensor for measuring the rate of flow of a fluid through a swirl flowmeter.
2. Prior Art
Prior art swirl flowmeter circuitry is responsive to a thermistor that is self-heated by an electric current. The source of the electric current is usually from a constant current source. As the precessing low pressure vortex, created by swirl means within the swirl meter and upstream from the probe, passes a probe port, a pneumatic blast impinges on the thermistor. This blast or puff of fluid cools the thermistor for a relatively short period of time. After the vortex passes the probe port, the thermistor temperature rises until the next vortex passes the probe port. This periodic cooling causes a cyclic variation in the temperature of the thermistor and is reflected in a periodic change in the thermistor resistance. As the resistance changes, the voltage drop across the thermistor changes and it is this resulting signal by which the precession frequency of vortex is determined. It is known that there is a well-defined relationship between the precession frequency of the vortex and the rate of flow of the fluid.
The output voltage from the thermistor is optimized by decreasing the size of the thermistor to maximize the surface to heat capacity ratio; by choosing a thermistor material having a high temperature coefficient of resistance; and by operating the thermistor at as high a current as possible within its thermal limitations. In prior art measuring systems, this results in an output voltage which is on the order of millivolts at the low end of fluid flow range of the meter and on the order of microvolts at the high end of the fluid flow range. Even though the amplitude of the pneumatic blast increases with the rate of flow, its duration decreases and for a given thermistor the temperature change which can be produced by such a short duration cooling blast is necessarily small.
This small temperature change of a given thermistor requires complex circuitry to generate a useful voltage signal. In applications such as used in a motor vehicle, shielding and power supply filtering are particularly difficult and the extraction of signal information from thermal noise involves bandpass filters, tracking filters, amplifiers, shaping circuits, automatic gain control circuits and other complex circuit units.